1. Field of the Invention
The present invention relates to an exhaust gas purifier for use in an internal combustion engine having, in an exhaust gas passage thereof, a catalyst device (NOx catalyst) which occludes NOx contained in the exhaust gas at a lean air-fuel ratio; i.e., in an oxygen-excessive atmosphere, and which releases occluded NOx at a stoichiometric air-fuel ratio or a rich air-fuel ratio; i.e., in a low-oxygen-concentration atmosphere.
2. Description of the Prior Art
Recently, in order to improve fuel economy, a lean-burn engine enabling combustion at a lean air-fuel ratio has been put into practical use. The lean-burn engine has a problem in that a conventional three-way catalytic converter cannot sufficiently purify NOx in an exhaust gas during a lean combustion due to its purification characteristics. Thus, recently there has been employed, for example, a catalyst device (NOx catalyst) for purifying NOx in the exhaust gas during the lean combustion through occlusion of NOx contained in the exhaust gas.
The NOx catalyst is known to purify NOx contained in exhaust gas in an oxygen-excessive atmosphere (at a lean air-fuel ratio) through occlusion of NOx onto a catalyst and to release the occluded NOx when an oxygen concentration lowers (at a stoichiometric air-fuel ratio or a rich air-fuel ratio). Specifically, in the oxygen-excessive atmosphere, the NOx catalyst produces a nitrate from NOx contained in exhaust to thereby occlude NOx. By contrast, in a low-oxygen-concentration atmosphere, the NOx catalyst causes the nitrate occluded on the NOx catalyst and CO contained in the exhaust gas to react with each other, thereby generating a carbonate and releasing NOx.
In the oxygen-excessive atmosphere during a lean operation, the NOx catalyst occludes NOx thereon. However, when the NOx catalyst becomes saturated with occluded NOx after continuation of the lean operation, most NOx contained in the exhaust gas is emitted into the atmosphere. Thus, before the NOx catalyst becomes saturated with NOx, the air-fuel ratio is switched to a stoichiometric air-fuel ratio or a rich air-fuel ratio to lower the oxygen concentration of exhaust gas, whereby NOx is released and reduced to thereby restore the NOx occlusion capability of the NOx catalyst. According to a technique disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 7-166913, when the air-fuel ratio of the engine is switched to a stoichiometric air-fuel ratio or a rich air-fuel ratio in order to restore the NOx occlusion capability of the NOx catalyst, the air-fuel ratio is gradually changed to a stoichiometric air-fuel ratio or a rich air-fuel ratio to thereby release and reduce NOx while suppressing a torque shock acting on the engine.
When NOx is released and reduced through switching the air-fuel ratio of the engine to the stoichiometric air-fuel ratio or the rich air-fuel ratio (CO is generated and supplied into the exhaust gas; i.e., to the NOx catalyst) in order to restore the NOx occlusion capability of the NOx catalyst, a portion of the supplied CO is consumed for releasing the occluded NOx, and a residual CO is consumed for reducing released NOx. When a ratio, at which NOx is reduced by means of reducers, such as the residual CO and HC, coincides with that at which NOx is released, release of NOx and CO into the atmosphere can be suppressed.
However, the technique disclosed in the above publication encounters a difficulty in establishing coincidence between a ratio at which NOx is reduced and that at which NOx is released. This is because the NOx occlusion capability restoration performance of the NOx catalyst; i.e., the releasability of the NOx catalyst with respect to the occluded NOx (NOx-releasing rate), depends on the form and amount of a catalytic component carried by the NOx catalyst.
In the case of employment of a NOx catalyst having improved NOx occlusion capability restoration performance, the NOx-releasing rate, or the rate at which NOx is released from the NOx catalyst, is also improved. As a result, the amount of NOx present in the exhaust gas and to be reduced by means of reducers tends to become smaller than that of NOx to be released (NOx to be reduced  less than NOx to be released). Thus, residual NOx which remains in the exhaust gas without being reduced is emitted into the atmosphere. By contrast, in the case of employment of an NOx catalyst having a limited NOx occlusion capability restoration performance, the amount of NOx to be reduced tends to become greater than that of NOx to be released (NOx to be reduced  greater than NOx to be released). Thus, reducers (CO, etc.) remain in the exhaust gas and are released into the atmosphere.
Generally, as the air-fuel ratio of the engine approaches the rich side (as the amount of CO increases), the NOx-releasing rate increases. Thus, when the air-fuel ratio is shifted toward the stoichiometric air-fuel ratio or the rich air-fuel ratio as described in the above publication, the NOx-releasing rate begins to increase at a near stoichiometric air-fuel ratio, at which the amount of CO begins to increase; thus, the amount of NOx to be released from the NOx catalyst increases. However, the amount of reducers (residual CO, HC, etc. which have not contributed to release of NOx) is not sufficient for reducing the increased amount of released NOx. As a result, the released NOx, remaining in exhaust gas, is released into the atmosphere without being reduced.
A conceivable solution to this problem is to increase the amount of reducers by enriching the air-fuel ratio of the engine. In this case, since the amount of CO serving as a reducer also increases, the amount of NOx to be released increases. Therefore, in actuality, the above-mentioned relation xe2x80x9cNOx to be reduced  less than NOx to be releasedxe2x80x9d relation remains unchanged. As a result, the residual NOx, which remains in exhaust without being reduced is released into the atmosphere, constituting failure to suppress emission of NOx.
Accordingly, the technique disclosed in the above publication encounters difficulty in establishing substantial coincidence between the amount of NOx to be reduced and that of NOx to be released and thus involves a problem in that exhaust gas characteristics are impaired during release of NOx from the catalyst and reduction of released NOx.
The present invention has been accomplished in view of the foregoing, and an object of the present invention is to provide an exhaust purifier for use in an internal combustion engine capable of reliably reducing NOx released from a catalyst device (NOx catalyst).
According to an exhaust purifier of the present invention for use in an internal combustion engine, when the exhaust air-fuel ratio of the engine is switched from a lean air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio, reducer-supplying means supplies a reducer for reducing NOx released from an NOx catalyst device to reduce NOx released from the catalyst device without impairing exhaust gas characteristics.
Since NOx released from the NOx catalyst device is reduced by means of the supplied reducer, NOx is not released into the atmosphere, thereby suppressing impairment in THE exhaust gas performance.
In the case of a cylinder-injection-type internal combustion engine having an injection valve for directly injecting fuel into a combustion chamber, the reducer-supplying means preferably injects fuel during an expansion stroke or an exhaust stroke subsequent to a main injection effected by the injection valve (injection during an intake stroke or injection during a compression stroke).
Thus, NOx released from the catalyst device can be reliably reduced without need of a complicated device.
According to the exhaust purifier of the present invention for use in an internal combustion engine, when NOx-releasing means is operated to create a low-oxygen-concentration exhaust atmosphere in order to release NOx from a NOx catalyst, the reducer-supplying means additionally supplies a reducer for reducing NOx released into an exhaust path, at a predetermined timing during operation of the NOx-releasing means, thereby reducing NOx released from the NOx catalyst without impairment of the exhaust gas characteristics.
Since NOx released from the catalyst device is reduced by means of the supplied reducer, NOx is not released into the atmosphere, thereby suppressing impairment in the exhaust gas performance.
Particularly, a preferred reducer to be supplied by the reducer-supplying means is mainly composed of, for example, HC, which does not increase the NOx releasing rate abruptly. Thus, in the case of, for example, a cylinder-injection-type internal combustion engine, the reducer-supplying means preferably assumes the form of fuel control for injecting additional fuel during an expansion stroke or an exhaust stroke subsequent to main injection (injection during an intake stroke or injection during a compression stroke) or assumes the form of an injection valve dedicated to injection of a reducer and adapted to inject fuel into an exhaust gas passage.
The NOx catalyst occludes NOx contained in the exhaust gas when the exhaust gas assumes a lean air-fuel ratio; i.e., the form of an oxygen-excessive atmosphere. The NOx catalyst releases occluded NOx when the exhaust gas assumes a stoichiometric air-fuel ratio or a rich air-fuel ratio; i.e., the form of a low-oxygen-concentration atmosphere. The reducer-supplying means operates at a predetermined period of time when the exhaust gas assumes a near stoichiometric air-fuel ratio as a result of operation of the NOx-releasing means. In the case of an engine having an injection valve for injecting fuel directly into a combustion chamber, the reducer-supplying means controls operation of the injection valve to additionally supply a reducer.
The NOx-releasing means has a regenerative function for establishing a rich air-fuel ratio in the exhaust gas for a first predetermined period of time and subsequently establishing a near stoichiometric air-fuel ratio for a second predetermined period of time when NOx occluded on the NOx catalyst is to be released. The reducer-supplying means operates when the NOx-releasing means causes switching of an air-fuel ratio in the exhaust gas to a rich air-fuel ratio. The exhaust purifier includes deterioration-detecting means for detecting the degree of deterioration of the NOx catalyst, and correction means for making correction to prolong the second predetermined period of time associated with the regenerative function of the NOx-releasing means or to shorten the operating period of time of the reducer-supplying means as the degree of deterioration of the NOx catalyst detected by the deterioration-detecting means increases.